The invention relates to imaging devices. More specifically, the invention relates to an exposure control for an optical scanner.
Desktop scanners typically use a step motor to move a charge coupled device ("CCD") across a sheet of paper or move a sheet of paper across the CCD. In a color copier or flatbed scanner, for instance, lines of pixels are generated as the CCD is moved over the sheet of paper. The CCD is moved at a constant velocity, and each line of pixels is given a constant exposure time. Therefore, the exact size of the pixels that make up a scanned image is a direct function of the motion accuracy of the drive system.
The step motor is controlled to a high positional accuracy, especially if scan stitching is performed. Two examples of scan stitching are as follows.
The first example involves color plane registration in a color scanner that collects red, green and blue information in a single pass. The CCD includes three rows of color photodiodes that are physically separated. A first row of photodioides detects the red information, a second row detects the green information, and a third row detects the blue information. The rows are physically separated on the CCD typically by 1/150 to 8/150 of an inch. After an image has been scanned, the information from each of the rows of photodiodes (that is, the color planes) are later realigned by electronics or software in increments of one pixel. Thus, the red color plane will later be shifted into alignment with the green color plane, and the blue color plane will later be shifted into alignment with the green color plane. Motion errors in between the rows of photodiodes and fractional pixel errors are difficult to correct after the image has been scanned. These residual position errors appear in the scanned image as "color fringes" around black-to-white edges.
The second example of stitch scanning involves stop/start repositioning of the CCD or paper sheet. Often the scanner will pause mid-scan because of a speed constraint such as memory being full, an I/O delay, a busy host computer or a slow print engine. If a speed constraint is encountered, the scanner stops scanning and waits for the constraint to clear. Then the CCD or paper is accelerated to scanning speed so that scanning can begin at the precise point where it had previously stopped. If the scanning does not begin at the precise point, or if the scanning speed has not been attained, position errors will appear in the image as discontinuities or "jaggies." Color fringes and jaggies become even more noticeable if the scanned image is enlarged or scan resolution is increased. Furthermore, color fringes and jaggies can severely reduce the accuracy of text recognition software, such as optical character recognition ("OCR") programs.
These two examples of scan stitching illustrate the need for positioning accuracy within 0.1 to 0.2 pixels. On a 1200 dpi scan, this requires an accuracy of 0.0001" (about two microns). However, inexpensive step motor drive systems used in commercial products such as flatbed scanners and color copiers are not capable of this level of accuracy.
There are other limitations associated with step motors. A step motor can be noisy, it has a limited range of speeds over which it can perform acceptably, and it consumes a lot of power. Additionally, motion of the step motor is inherently oscillatory because it is driven by discrete step inputs. The oscillatory motion can cause color fringes in scanners using three-channel RGB CCDs.
Scan motion accuracy is often limited by the finite step size. Even if the step motor is microstepped to improve resolution and reduce torque ripple, some motion errors are still inevitable.
An inexpensive step motor drive system does not have position or velocity feedback, so there is no way to know the true position of the CCD or whether the step motor has stalled. Therefore, the step motor is typically overdriven with a considerable amount of extra torque to satisfy the worst case load. Overdriving increases the size, cost and heat dissipation of the motor driver.
It would be desirable to replace the step motor with a dc motor. However, control of a dc motor has heretofore not been accurate enough to produce a quality image and meet the cost, size, weight, and power requirements of a low cost commercial scanning product such as a flatbed scanner, scrollfed scanner, color copier, fax machine or all-in-one multi-function product.